drm/i915/gvt: Use common error handling code in shadow_workload_ring_buffer()

[Dan Carpenter]

The point of unwind code is to undo what was done earlier.  If a
function allocates a list of things, using standard unwind style makes
it simpler, safer and more readable.

This isn't the case here.  Instead of making the code more readable,
we're making it more convoluted.  It's just that two out of three error
messages happened to be the same and Markus wants to save a bit of
memory by using the same string.  The memory savings is not so big that
it's worth making the code less readable.

regards,
dan carpenter